Large diameter pipes which carry viscous, abrasive slurries such as tar sands, exploited for oil extraction and recovery, must endure a harsh working environment. As noted, the tar sands are abrasive and are pumped through the pipes at pressures up to 600 psi and temperatures in excess of 90° C. The pipes are outfitted with insulating and abrasive resistant liners to mitigate pipe corrosion and inhibit heat transfer from the slurry to the ambient which would otherwise render the tar sands too viscous to pump effectively. The pipes must permit large mass flow rates and must withstand the aforementioned high temperatures and pressures. Consequently, the pipes are preferably formed of steel and have large diameters, thick side walls and tend to be heavy. Diameters up to 30 inches are not unusual, as well as weights on the order of 160 lbs/foot of pipe stock.
Even with the abrasion resistant liners the pipes tend to wear, and the wear is uneven, with the lowermost third of the pipe inside surface wearing most rapidly. This is due to the fact that the particles in the slurry settle to the bottom of the flow stream due to gravity and they tend to more rapidly abrade the surface over which they pass. The pipe segments must be replaced when worn out, or, to extend their life, the pipes segments are disconnected, rotated about their long axis to position a relatively un-abraded surface on the bottom, and then reconnected. Either way significant maintenance is required.
During assembly or servicing of a pipe line, segments of several pipes are joined together and then lifted and maneuvered into position with a crane for connection to the pipe line. Due to the high weight of the pipe segments significant bending stresses are imposed on the joints, which must be strong enough to maintain joint integrity.
To obtain the required strength and rigidity at pipe joints, such pipes have, in the past, been joined by welding. However, this is an expensive process which requires skilled labor and time consuming welding procedures. It is considered advantageous to use mechanical couplings instead of welded joints. Mechanical couplings simplify the procedure and do not require the time or the skills needed to form a welded joint. They facilitate rapid assembly and disassembly for servicing to replace or reposition worn pipes.
The preferred mechanical couplings may be formed of segments having keys which engage circumferential grooves near the ends of the pipes. The couplings have an internal gasket or seal and are bolted around the pipe ends in end-to-end relation, the keys being forcibly engaged with the grooves to effect a rigid joint, the seal operating to ensure fluid-tight integrity.
Like the pipes, the couplings must be structurally strong and rigid to join the heavy pipes and seal them effectively against the high pressures. The couplings are also designed to take the bending stresses imposed when a sub-assembly of multiple pipe segments is lifted and maneuvered into position in a pipe line. The couplings, therefore, tend to be massive. The weight and size of the pipes and their couplings make them awkward to handle when joining the pipes in end-to-end relation using the couplings. Due to the weight and awkwardness, it is physically difficult, even using lifting machinery such as hydraulic cranes and the like, to position pipe elements in proper end-to-end spacing and engage the keys with the grooves. There is clearly a need for mechanical couplings which, despite their size and weight, may be applied to the pipe elements so that their keys readily engage the grooves in the pipe ends and which have the required stiffness and strength to effect a rigid, fluid-tight joint.